Coherent laser beam sources have many industrial applications—for example in communication systems, in information processing, and in holographic displays. There are two previously known types of one-dimensional (1D) photonic band gap (PBG) structures: (1) periodic layered media, and (2) cholesteric liquid crystals (CLCs). In both of these systems the wavelength inside the medium at the center of the band gap is twice the period of the structure in question. In CLC structures, the band gap exists only for the circular polarized component of light, which has the same sense of rotation as the structure. The second circular component is unaffected by the structure. The first type of structure has been implemented in optical fibers and is known as a fiber Bragg grating (FBG). However, the second type of structure—CLCs—does not exist in the form of fibers. Fiber Bragg gratings have many applications—fiber components form the backbone of modern information and communications technologies and are suitable for a wide range of applications—for example in information processing and especially in optical fiber communication systems utilizing wavelength division multiplexing (WDM). However, FBGs based on conventional periodic structures are not easy to manufacture and suffer from a number of disadvantages. Similarly, other types of desirable fiber gratings are difficult to fabricate using previously known techniques.
The conventional method of manufacturing fiber gratings (including FBGs) is based on photo-induced changes of the refractive index. One approach requires fine alignment of two interfering laser beams along the length of the optical fiber. Extended lengths of periodic fiber are produced by moving the fiber and re-exposing it to the interfering illumination while carefully aligning the interference pattern to be in phase with the previously written periodic modulation. The fiber core utilized in the process must be composed of specially prepared photorefractive glass, such as germanium doped silicate glass. This approach limits the length of the resulting grating and also limits the index contrast produced. Furthermore, such equipment requires perfect alignment of the interfering lasers and exact coordination of the fiber over minute distances when it is displaced prior to being exposed again to the laser interference pattern.
Another approach to fabricating fiber gratings involves the use of a long phase mask placed in a fixed position relative to a fiber workpiece before it is exposed to the UV beam. This approach requires photosensitive glass fibers and also requires manufacture of a specific mask for each type of fiber grating produced. Furthermore, the length of the produced fiber is limited by the length of the mask unless the fiber is displaced and re-aligned with great precision. This restricts the production of fiber gratings to relatively small lengths making the manufacturing process more time consuming and expensive.
One novel approach that addressed the problems in fabrication techniques of previously known fiber gratings is disclosed in the commonly-assigned co-pending U.S. patent application entitled “Apparatus and Method for Manufacturing Periodic Grating Optical Fibers”. This approach involved twisting a heated optical preform (comprising either a single fiber or multiple adjacent fibers) to form a chiral structure having chiral fiber grating properties. Another novel approach for fabricating chiral fibers having chiral fiber grating properties, disclosed in the commonly-assigned co-pending U.S. provisional patent application entitled “Apparatus and Method for Fabricating Helical Fiber Bragg Gratings”, involved heating and twisting optical fibers having various core cross-section configurations or composed of different dielectric materials, inscribing patterns on the outer surface of the fiber cores, and optionally filling the patterns with dielectric materials.
It would thus be desirable to provide an advantageous fiber grating that has superior properties and that is easy to fabricate.